linux-kernel - Re: [PATCH] memcg: use ID in page

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Message-Id: <20100824105405.abf226e6.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Tue, 24 Aug 2010 10:54:05 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Daisuke Nishimura <nishimura@....nes.nec.co.jp>
Cc:	linux-mm@...ck.org,
	"balbir@...ux.vnet.ibm.com" <balbir@...ux.vnet.ibm.com>,
	gthelen@...gle.com, m-ikeda@...jp.nec.com,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	kamezawa.hiroyuki@...il.com
Subject: Re: [PATCH] memcg: use ID in page_cgroup

On Tue, 24 Aug 2010 10:14:25 +0900
Daisuke Nishimura <nishimura@....nes.nec.co.jp> wrote:

> > > > @@ -723,6 +729,11 @@ static inline bool mem_cgroup_is_root(st
> > > >  	return (mem == root_mem_cgroup);
> > > >  }
> > > >  
> > > > +static inline bool mem_cgroup_is_rootid(unsigned short id)
> > > > +{
> > > > +	return (id == 1);
> > > > +}
> > > > +
> > > It might be better to add
> > > 
> > > 	BUG_ON(newid->id != 1)
> > > 
> > > in cgroup.c::cgroup_init_idr().
> > > 
> > 
> > Why ??
> > 
> Just to make sure that the root css has id==1. mem_cgroup_is_rootid() make
> use of the fact.
> I'm sorry if I miss something.
> 

Hmm. The function allocating ID does

4530 static struct css_id *get_new_cssid(struct cgroup_subsys *ss, int depth)
4531 {
==
4546         spin_lock(&ss->id_lock);
4547         /* Don't use 0. allocates an ID of 1-65535 */
4548         error = idr_get_new_above(&ss->idr, newid, 1, &myid);
4549         spin_unlock(&ss->id_lock);
==

and allocates ID above "1", always.

Adding BUG_ON(newid->id != 1) will mean that we doubt the bitmap function and
consider possibility that new->id == 0.

But, we're 100% sure that it never happens.

I don't think adding a comment is a right thing to do.

Thanks,
-Kame

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